Bacterial capsular polysaccharides have been widely used in immunology for many years for the prevention of bacterial disease. A problem with such a use, however, is the T-independent nature of the immune response. These antigens are thus poorly immunogenic in young children. This problem has been overcome through conjugating the polysaccharide antigens to a carrier protein (a source of T-helper epitopes) which may then be used to elicit a T-dependent immune response, even in the first year of life.
Various conjugation techniques are known in the art. Conjugates can be prepared by direct reductive amination methods as described in, US200710184072 (Hausdorff) U.S. Pat. No. 4,365,170 (Jennings) and U.S. Pat. No. 4,673,574 (Anderson). Other methods are described in EP-0-161-188, EP-208375 and EP-0-477508. The conjugation method may alternatively rely on activation of hydroxyl groups of the saccharide with 1-cyano-4-dimethylamino pyridinium tetrafluoroborate (CDAP) to form a cyanate ester. Such conjugates are described in PCT published application WO 93/15760 Uniformed Services University and WO 95/08348 and WO 96/29094. See also Chu C. et al Infect. Immunity, 1983 245 256.
Reductive amination involves two steps, (1) oxidation of the antigen, (2) reduction of the antigen and a carrier protein to form a conjugate. The oxidation step may involve reaction with periodate, however oxidation by periodate may lead to size reduction (WO94/05325).